The present invention relates to an exhaust emission control system for an internal combustion engine, and particularly to an exhaust emission control system which includes an NOx removing device for removing NOx (nitrogen oxides) and an oxygen concentration sensor arranged downstream of the NOx removing device, and determines deterioration of the NOx removing device and abnormality of the oxygen concentration sensor.
An exhaust emission control system in which a three-way catalyst for purifying exhaust gases is arranged in an exhaust system of an internal combustion engine and two oxygen concentration sensors are respectively provided upstream and downstream of the three-way catalyst is well known. In this system, an air-fuel ratio of an air-fuel mixture to be supplied to the engine is feedback-controlled according to outputs of the two oxygen concentration sensors. With respect to this exhaust emission control system, a method for determining abnormality of the downstream oxygen concentration sensor is disclosed in Japanese Patent Publication No. 2826564.
In this abnormality determining method, the air-fuel ratio is changed to the rich region with respect to the stoichiometric ratio and is held in the rich region when an output of the oxygen concentration sensor arranged downstream of the three-way catalyst exhibits a lean air-fuel ratio. If the output of the downstream oxygen concentration sensor exhibits a lean air-fuel ratio over a predetermined time period, while the output of the oxygen concentration sensor arranged upstream of the three-way catalyst exhibits a rich air-fuel ratio, it is determined that the downstream oxygen concentration sensor is abnormal.
When the air-fuel ratio of the air-fuel mixture supplied to an internal combustion engine is set to the lean region with respect to the stoichiometric ratio, that is, when a so-called lean operation is carried out, the amount of NOx exhausted from the engine tends to increase. To cope with the NOx, a technique for reducing exhaust emission has been known, in which an NOx removing device containing an NOx absorbent for absorbing NOx is provided in an exhaust system of the engine, to thereby reduce the exhaust emission. Another technique is known where an oxygen concentration sensor provided downstream of an NOx removing device determines the deterioration of the NOx removing device according to an output of the downstream oxygen concentration sensor (Japanese Patent Laid-open No. Hei 10-299460).
In the operation of an engine having an NOx removing device, a lean operation is carried out for a long period of time. Therefore, the output of an oxygen concentration sensor arranged downstream of the NOx removing device, in the exhaust system, exhibits a lean air-fuel ratio for a long period of time. Further, when the air-fuel ratio is set to a rich region, NOx absorbed in the NOx removing device is reduced.
Accordingly, if the technique disclosed in the above document, Japanese Patent Publication No. 2826564, is applied to the oxygen concentration sensor arranged downstream of the NOx removing device, it is difficult to appropriately execute the abnormality determination at a suitable timing when the output of the oxygen concentration sensor changes. Specifically, in order to positively change an output of an oxygen concentration sensor provided downstream of an NOx removing device to a rich region, it may often required to continue to operate in the rich region over a long period of time. This may cause an adverse effect on the exhaust emission characteristic and the engine drivability.
Further, in the operation of an engine having an NOx removing device, a large proportion of time is spent in lean operation. Therefore, it may be expected that an abnormality in a downstream oxygen concentration sensor can be rapidly determined.
It is accordingly an object of the present invention to provide an exhaust emission control system for an internal combustion engine that is capable of executing abnormality determination of an oxygen concentration sensor provided downstream of an NOx removing device with suitable timing, to suppress the adverse effects of the abnormality determination on the exhaust emission characteristic and the engine drivability.
To achieve the above object, according to an aspect of the present invention, there is provided an exhaust emission control system for an internal combustion engine, including: nitrogen oxides removing means provided in an exhaust system of the engine for removing nitrogen oxides in exhaust gases in a condition where an air-fuel ratio of an air-fuel mixture supplied to the engine is set to a lean region with respect to a stoichiometric ratio; an oxygen concentration sensor provided downstream of the nitrogen oxides removing means for detecting the concentration of oxygen in the exhaust gases; deterioration determining means for determining deterioration of the nitrogen oxides removing means on the basis of an output of the oxygen concentration sensor after the air-fuel ratio has been changed from the lean region to a rich region with respect to the stoichiometric ratio; and abnormality determining means for determining abnormality of the oxygen concentration sensor on the basis of an output of the oxygen concentration sensor during a period in which the air-fuel ratio is kept in the rich region with respect to the stoichiometric ratio immediately after execution of the deterioration determination by the deterioration determining means.
With this configuration, the deterioration determination of the nitrogen oxides removing means is executed on the basis of the output of the oxygen concentration sensor downstream of the nitrogen oxides removing means after the air-fuel ratio of an air-fuel mixture to be supplied to the engine has been changed from the lean region to the rich region, and the abnormality of the oxygen concentration sensor is determined on the basis of the output of the oxygen concentration sensor during a period in which the air fuel-ratio is kept in the rich region with respect to the stoichiometric ratio immediately after execution of the deterioration determination. Since the air-fuel ratio is changed to the rich region by executing the deterioration determination of the NOx removing means and thereby the amount of NOx absorbed in the NOx removing means is reduced, the oxygen concentration on the downstream side of the NOx removing means can be positively lowered by keeping the air-fuel ratio in the rich region only for a very short period of time after the deterioration determination of the NOx removing means. Accordingly, the abnormality determination of the oxygen concentration sensor can be accurately performed by monitoring the output of the oxygen concentration sensor in the above very short period of time in which the air-fuel ratio is kept in the rich region after the deterioration determination. That is, by performing the abnormality determination of the oxygen concentration sensor immediately after the deterioration determination of the NOx removing means, it is possible to minimize the period of time in which the enrichment of the air-fuel ratio is kept for the abnormality determination, and hence to suppress the adverse effect of the enrichment on the exhaust emission characteristic and the drivability of the engine.
In the above exhaust emission control system, preferably, the abnormality determination by the abnormality determining means is executed if the output of the oxygen concentration sensor is unchanged before the end of the deterioration determination by the deterioration determining means.
With this configuration, the abnormality determination by the abnormality determining means is executed if the output of the oxygen concentration sensor is unchanged before the end of the deterioration determination by the deterioration determining means. In other words, if the output of the oxygen concentration sensor has changed before the end of the deterioration determination by the deterioration determining means, it can be determined that the oxygen concentration sensor is normal. Accordingly, by executing the abnormality determination when the output of the oxygen concentration sensor is unchanged until the end of the deterioration determination, it is possible to minimize the enrichment of the air-fuel ratio for the abnormality determination.
The abnormality determining means, preferably, determines that the oxygen concentration sensor is abnormal if the output of the oxygen concentration sensor is not changed from a value indicative of a lean air-fuel ratio to a value indicative of a rich air-fuel ratio within a predetermined period when the enrichment of the air-fuel ratio is continued over the predetermined period immediately after the end of the deterioration determination.
The exhaust emission control system, preferably, further includes determination inhibiting means for inhibiting the abnormality determination by the abnormality determining means immediately after starting of the engine.
According to another aspect of the present invention, there is provided an exhaust emission control system for an internal combustion engine, including: nitrogen oxides removing means provided in an exhaust system of the engine for removing nitrogen oxides in exhaust gases in a condition where an air-fuel ratio of an air-fuel mixture supplied to the engine is in a lean region with respect to a stoichiometric ratio; an oxygen concentration sensor provided downstream of the nitrogen oxides removing means for detecting the concentration of oxygen in the exhaust gases; deterioration determining means for determining deterioration of the nitrogen oxides removing means on the basis of an output of the oxygen concentration sensor after the air-fuel ratio has been changed from the lean region to a rich region with respect to the stoichiometric ratio; and abnormality determining means for determining that the oxygen concentration sensor is abnormal if a condition where the output of the oxygen concentration sensor indicates that the air-fuel ratio is in the rich region with respect to the stoichiometric ratio continues before execution of the deterioration determination by the deterioration determining means.
With this configuration, the deterioration determination of the nitrogen oxides removing means is executed on the basis of the output of the oxygen concentration sensor downstream of the nitrogen oxides removing means after the air-fuel ratio of an air-fuel mixture supplied to the engine changes from the lean region to the rich region with respect to the stoichiometric ratio, and it is determined that the oxygen concentration sensor is abnormal if a condition where the output of the oxygen concentration sensor is in the rich region with respect to the stoichiometric ratio is continued before execution of the deterioration determination by the deterioration determining means. Since the oxygen concentration sensor is inactive at the time of cold starting of the engine, the output of the oxygen concentration sensor indicates a rich air-fuel ratio. On the other hand, the deterioration determination of the nitrogen oxides removing means is executed when the amount of NOx absorbed in the nitrogen oxides removing means has reached a predetermined amount. Accordingly, unless the output of the oxygen concentration sensor changes to a value indicative of a lean air-fuel ratio before execution of the deterioration determination, it can be determined that the oxygen concentration sensor is abnormal. As a result, it is possible to perform the abnormality determination of the oxygen concentration sensor before execution of the deterioration determination of the nitrogen oxides removing device, to thereby quickly determine the abnormality of the oxygen concentration sensor and eliminate the need of continuing the enrichment of the air-fuel ratio immediately after execution of the deterioration determination. This is effective to eliminate the adverse effects of enrichment on the exhaust emission characteristic and drivability of the engine.
The exhaust emission control system, preferably, further includes determination inhibiting means for inhibiting the abnormality determination by the abnormality determining means immediately after starting of the engine.
According to a further aspect of the present invention, there is provided an exhaust emission control system including: nitrogen oxides removing means provided in an exhaust system of the engine for removing nitrogen oxides in exhaust gases in a condition where an air-fuel ratio of an air-fuel mixture supplied to the engine is in a lean region with respect to a stoichiometric ratio; a downstream oxygen concentration sensor provided downstream of the nitrogen oxides removing means for detecting the concentration of oxygen in the exhaust gases; enrichment means for setting the air-fuel ratio to a rich region with respect to the stoichiometric ratio when a condition where the output of the downstream oxygen concentration sensor indicates that the air-fuel ratio is in the lean region with respect to the stoichiometric ratio continues after the end of starting of the engine; abnormality determining means for determining abnormality of the downstream oxygen concentration sensor on the basis of the output of the downstream oxygen concentration sensor during execution of the air-fuel enrichment by the enrichment means; and lean operation inhibiting means for inhibiting that the air-fuel ratio is set to the lean region with respect to the stoichiometric ratio until the end of the abnormality determination of the downstream oxygen concentration sensor.
With this configuration, the air-fuel ratio is set to the rich region with respect to the stoichiometric ratio if a condition where the output of the downstream oxygen concentration sensor indicates that the air-fuel ratio is in the lean region continues after the end of starting of the engine, and the abnormality of the oxygen concentration sensor is determined on the basis of the output from the oxygen concentration sensor during execution of the enrichment of the air-fuel ratio. In addition, the setting of the air-fuel ratio to the lean region with respect to the stoichiometric ratio is inhibited until the abnormality determination ends. Accordingly, the abnormality determination of the oxygen concentration can be performed before starting of the lean operation, that is, before the amount of NOx absorbed in the NOx removing device increases after starting of the engine, so that it is possible to minimize the enrichment execution period of time for the abnormality determination, to thereby minimize the adverse effect of enrichment on the exhaust emission characteristic and the drivability of the engine.
The enrichment of the air-fuel ratio by the enrichment means is, preferably, allowed after a vehicle on which the engine is mounted begins running.
The exhaust emission control system, preferably, further includes: an upstream oxygen concentration sensor provided upstream of the nitrogen oxides removing means for detecting the concentration of oxygen in the exhaust gases; and deterioration determining means for determining deterioration of the nitrogen oxides removing means on the basis of an output of the downstream oxygen concentration sensor after the air-fuel ratio has been changed from the lean region to a rich region with respect to the stoichiometric ratio. In this system, the deterioration determining means comprises deterioration determination enrichment means for switching the air-fuel ratio from the lean region to the rich region with respect to the stoichiometric ratio, and a reducing component amount calculating means for calculating an amount of reducing components flowing into the nitrogen oxides removing means from the time when the output of the upstream oxygen concentration sensor has changed to a value indicative of a rich air-fuel ration after starting of the enrichment of the air-fuel ratio by the abnormality determination enrichment means. The deterioration determining means determines deterioration of the nitrogen oxides removing means on the basis of the amount of the reducing components calculated by the reducing component amount calculating means and the output of the downstream oxygen concentration sensor.
Preferably, the deterioration determining means determines that the nitrogen oxides removing means is deteriorated if the output of the downstream oxygen concentration sensor indicates a rich air-fuel ratio when the amount of the reducing components has reached a predetermined amount.
The reducing component amount calculating means, preferably, calculates the amount of the reducing components by integrating the amount of the exhaust gases flowing in the nitrogen oxides removing means.
The reducing component amount calculating means, preferably, uses a basic fuel amount, which is set so that the air-fuel ratio becomes a constant value according to a rotational speed and an absolute intake pressure of the engine, as a parameter indicative of the amount of exhaust gases flowing into the nitrogen oxides removing device.
Other objects and features of the invention will be more fully understood from the following detailed description and appended claims when taken with the accompanying drawings.